Direct reduction of iron oxide and other metallic oxides may be conducted in rotary hearth furnaces ("RHF") using pelletized or briquetted feed deposited upon a rotating hearth. Briefly, a RHF is a continuous reheating furnace generally having an annular inner wall circumscribed by a spaced annular outer wall. The space there between includes a circular rotating hearth. Burners may be installed in the inner and outer walls and in the roof Gases from the furnace are permitted to vent through a flue located in the roof.
Material is usually loaded (dropped) onto the rotating hearth by a conveyor or chute. After the material is conveyed along the hearth path it is removed by a discharge auger. A discharge auger typically consists of a central shaft with solid helical metal flights welded thereto projecting away from the central shaft.
Due to the corrosive nature of the gases and materials present within the RHF, coupled with the high temperatures therein, the discharge auger is susceptible to frequent failure. In particular, because of the harsh environment within the furnace the metal flights generally deteriorate. High temperatures and the presence of oxygen or one or more of sodium, sulfides, chlorides, fluorides, potassium, lead, zinc, tin, iron, nickel and chromium within the RHF oftentimes corrodes and erodes the auger and renders the auger ineffective.
To lessen the effects of the high temperatures (1300.degree.-2300.degree. F.)(704.degree.-1260.degree. C.) involved, a cooling fluid is frequently passed through the auger. See U.S. Pat. Nos. 3,443,931 and 4,636,127, incorporated herein by reference. It is intended that with sufficient flow rate, the fluid cooling maintains the central shaft of the auger within safe operating temperatures. However, it will be appreciated that the helical tight of the discharge auger also receives considerable heat from both the furnace and the material in the furnace. Although some heat is conducted through the helical flight to the fluid cooled central shaft, the high radiation heat transfer from the furnace and the limited thermal conductivity of the helical flight often causes the tips of the helical flight to operate over the maximum desired operating temperature of the metal alloys forming the helical flight thereby leading to premature auger failure.
It will be appreciated that failure of the auger necessitates replacement of the auger and unwanted frequent downtime, high maintenance and labor costs, and inefficient use of the furnace which, in turn, leads to higher unit costs. In view of the foregoing, it will be appreciated that there is a significant need for an improved furnace discharge assembly.
An object of the present invention is to provide an improved furnace discharge assembly. Another object of the present invention is to provide a furnace discharge assembly including a discharge auger capable of better withstanding the high operating temperatures of a furnace. Yet another object of the present invention is to provide a furnace discharge assembly including a discharge auger and a fluid cooled hood to act as a heat sink to maintain the heat resistance of the metal alloy helical flights at an acceptable operating temperature. Still another object of the present invention is to provide a furnace discharge assembly that is simple and economical to manufacture and/or operate.